This invention relates to browning vessels which are used together with microwave ovens, and, in particular, to improved vessels having a relatively wider bottom wall thereof and enabling to generally uniformly brown foodstuff received on the wider bottom wall.
To cook foodstuff in the use of a microwave oven, a vessel of heat resistant materials, such as a glass, glass ceramic, or ceramic vessel is used for containing foodstuff to be cooked. The vessel containing foodstuff is placed within a cooking chamber of the microwave oven to cook foodstuff.
The microwave cooking is advantageous in that foodstuff is rapidly and efficiently processed, but has a disadvantage that the surface of the cooked foodstuff is not browned.
To brown the surface of the cooked foodstuff, a browning vessel has been used, which is provided with an electroconductive film, or a tin oxide coating, on the lower surface of the bottom wall of a glass, glass ceramic, or ceramic vessel, or dish. The electroconductive film generates heat by internal currents generated by the microwave energy, and the generated heat is radiated to, and conducted to, the surface of foodstuff received on the upper surface of the bottom wall of the vessel to brown the surface of the cooked foodstuff.
In known browning vessels, a single electroconductive film with a predetermined pattern is coated on the lower surface of the bottom wall of the vessel, as is disclosed in U.S. Pat. No. 3,965,323.
The U.S. patent purposes to displace the electroconductive film coated on the lower surface above the lowermost support surface of the vessel and to form a bottom-open recess in the vessel, to promote a uniform heating of the coated area thus facilitating the utilization of larger areas providing uniform browning.
But, even if the proposal by the U.S. patent is employed, a uniform browning is not achieved, for example, by heating for a usual cooking time about 6-8 minutes using a microwave oven of a rated power of 600 watts, if the area of the bottom wall of the vessel is more than 450 cm.sup.2, specifically in the vessel having a relatively wider bottom wall of an area of such as 550 cm.sup.2 or more.
We found out following facts as to browning vessels having an electroconductive film on a lower surface of a bottom wall thereof, through our strict examination; (1) it is necessary for uniformly browning foodstuff that at least 60% of the upper surface area of the bottom wall of the vessel is elevated to a temperature of about 230.degree. C. or more, (2) if the area of the electroconductive film is more than 250 cm.sup.2, the browning temperature of about 230.degree. C. is not obtained, (3) the temperature is readily elevated to a higher level at a peripheral portion of the electroconductive film, strictly stated at an outside and inside portion extending within 5 mm from the edge of the electroconductive film, than the other portion, and (4) if the electroconductive film has a sharp corner, the portion of the sharp corner is readily elevated to a higher portion than the other portion.
This invention is based on these our new knowledge.